Pretreatment of chips with white liquor prior to a treatment with black liquor

ABSTRACT

The method is for the manufacture of cooked cellulose pulp in which the starting material, preferably chips, undergoes a successive increase in temperature towards cooking temperature. This is done while the chips are first treated in a pre-treatment zone in which the main part, greater than 50%, of fresh white liquor necessary for the cooking stage is added, after which this alkali-rich treatment fluid is withdrawn and replaced to a major extent by black liquor. The alkali-rich treatment fluid that has been withdrawn after the pre-treatment stage is then added to the cooking stage, which is why the addition of fresh white liquor to the cooking stage is reduced to an equivalent amount. Maximal yield-enhancing effect is obtained using poly-sulphide-rich white liquor during the pre-treatment at a temperature in the interval 60±40° C. and retention time in the interval 2-60 minutes, preferably 2-10 minutes.

The present invention concerns a method for the production of cellulosepulp according to the introduction of claim 1.

THE PRIOR ART

In older conventional continuous digesters, all alkali was essentiallyadded at the inlet or at the top of the digester during uninterruptedand established operation. A certain addition of alkali also took placein the feed system, the main purpose of which was to lubricate thehigh-pressure tap and, to a certain extent, to adjust the level ofalkali. In addition, a certain addition may also have taken place at thebottom of the digester, but in this case principally in order todissolve temporary blockages or in order to initiate the process duringstart-up.

Extremely high concentrations of alkali at a level of 60 grams NaOH perlitre of cooking fluid, or higher, were obtained at the top of thedigester, and relatively low levels of residual alkali were obtained inthe extracted expended cooking fluid. The white liquor charge wassubsequently divided further so that more white liquor was added duringimpregnation, in particular for two-vessel cooking systems, in order toreduce the high concentrations of alkali at the beginning of the cookingstage.

Cooking technology has since then undergone development, the purpose ofwhich has been to achieve increased yield and improved pulp quality. Animportant precondition was the requirement to limit high levels ofalkali, which could have a detrimental influence on the quality of thepulp, and instead focus at achieving a constant level of alkali duringthe cooking stage. ITC (IsoThermal Cooking) technology is one method ofimproving the quality of the pulp. In this case, the cooking temperatureis held at a constant level during the complete cooking stage,preferably during a longer part of the retention time in the digesterthan that used in known technology, and in which alkali is added at theend of the cooking stage. The cooking temperature could in this way bereduced to a lower level, and the division of the addition of alkali tothe cooking stage ensured lower levels of alkali at the beginning of thecooking stage, a result that was advantageous for the quality of thepulp.

The initial level of alkali during the cooking stage has been reduced inthe MCC (Modified Continuous Cooking) technology, and an alkali level ofapproximately 23 g NaOH/I is typically established in the transfer tothe digester, after which a cooking zone with a level that typicallylies around 15 g NaOH/I is established, with a final withdrawal from thedigester at a level of approximately 10 g NaOH/I.

Addition of alkali in all of these concepts has occurred early duringthe cooking stage, or early during impregnation, after which theestablished cooking fluid has successively received the addition ofgreater or lesser amounts of alkali.

The MCC technology was developed during the 1980s and involves thedivision of alkali charges. The main part, approximately 75%-80%, wasnormally added in the concurrent zone, 45%-70% of which was added to theimpregnation stage, and 10%-35% to the cooking zone, while the remainingamount, approximately 20%-25% was added to the countercurrent zone. Theconcentration of alkali could in this way be reduced to a level ofapproximately 40 g NaOH/I at the commencement of the impregnation. Acertain evening out of the alkali profile during the cooking stage couldin this way be achieved.

A total charge of alkali added to the cooking stage can typically beequivalent to 18% effective alkali, calculated as NaOH, in a digesterusing the MCC technology. Of this, 12% is added to the impregnation (therelative fraction then is 12/18*100≈67% of the total charge), while 2%(relative fraction 2/18*100≈11% of the total charge) is added to thetransfer line, and 4% (equivalent to a relative fraction of 4/18*100≈22%of the total charge) to the second countercurrent zone during thecooking stage.

The use of very high fluid/wood ratios has been introduced, as has theuse of a high fraction of black liquor in the pre-impregnation vessel,in order to further even out the alkali profile during the cookingstage. This technology constitutes one of the basic principles of theCOMPACT COOKING™ concept developed by Kvaemer Pulping. The alkaliconcentration in the cooking fluid can in this way be reduced, while agreat deal of alkali is at the same time available in the impregnationfluid and in the cooking fluid during the initial and rapidneutralisation process. The amount of alkali required for an efficientneutralising process can then be present in the cooking fluid.Fluid/wood ratios as high as 7:1 and up to 8:1 have been applied in thepreimpregnation vessels in these systems and in digesters with anintegrated impregnation zone.

Various suggestions for the adjustment of alkali during the cookingstage in the digester have been used with the purpose of evening out thealkali profile. For example, adjustment circuits can be used, in whichan amount of cooking fluid is withdrawn from the digester andsubsequently returned to the digester following adjustment of thealkali, or in which the cooking fluid that is withdrawn and subsequentlyreturned to the digester is fully or partially replaced by solvent, aprocedure that principally gives a reduction in the dissolved material(lignin, etc.). Withdrawal of the cooking fluid at several positions andthe subsequent replacement of the withdrawn cooking fluid by anotherfluid, however, results in a reduced yield, since residual fibres anddissolved hemicellullose disappear with the cooking fluid that iswithdrawn.

One method of increasing the yield has been to add polysulphide duringthe cooking stage, although the polysulphide is liable to thermaldegradation, something that leads to a large amount being degraded bythe high cooking temperature before any influence to increase the yieldof the pulp is achieved.

A further method involves the return of cooking fluid that has beenwithdrawn from the initial phase of the cooking stage, such cookingfluid being rich in dissolved xylan, and the subsequent return of thexylan-rich fluid to the final phase of the cooking stage, where thexylan can be re-precipitated onto the fibres. This process depends onthe xylan-rich liquor being retained for a long period, typically atleast 60 minutes, such that the precipitation process has sufficienttime to commence and to give a noticeable influence on the increase inyield. The yield can typically be increased by 2%-5% using this type ofxylan re-precipitation.

The yield is highly significant during cooking since an increase of only1% means that a production facility of normal size, having an output of1,500 tonnes a day, would experience an increase in production of 15tonnes, which, with a pulp price of 700 USD/ADT, gives an increasedincome of at least 10,500 USD a day. Increase of marginal productionresults in essentially pure profit.

Furthermore, the load on the recovery system is reduced if a smallerfraction of the cellulose is sent to evaporation and combustion in thesoda recovery furnace.

Several different solutions are known in which black liquor is used asimpregnation fluid in an impregnation zone before the cooking stage. Asystem is revealed by U.S. Pat. No. 5,080,755 with black liquor in theinlet. A variation is revealed by U.S. Pat. No. 5,053,108 in which blackliquor withdrawn from the digester is recirculated to the high-pressuretap in order there to form the major part of the treatment fluid in thetransfer circulation to the digester. EP 477,059 reveals a variant thathas been developed further, in which wood chips impregnated with blackliquor are raised to cooking temperature before the principal additionof the white liquor. These show that numerous suggestions for processeshave been studied with the purpose of improving the quality of the pulpand the yield while maintaining a high degree of delignification in thepulp that is washed after the cooking stage.

Aim and Purpose of the Invention

The principal purpose of the invention is to increase the yield from thecooking stage by using a sequence of treatment fluids in which the mostadvantageous conditions possible for the pulp are established during thetransfer, the impregnation and the subsequent cooking.

A second purpose is to exploit during the use of primarilypolysulphide-rich white liquor the effect of polysulphide in raising theyield at a process position at which the temperature is not extremelyhigh and at which the polysulphide can provide its yield-increasingeffect, without giving the polysulphide sufficient time to be degraded.A subsequent increase in temperature of the chips can then take place inanother impregnation fluid and cooking fluid, after which the cookingstage can properly take place. The polysulphide reacts more rapidly thanalkali, in particular at low temperatures, and essentially only a minorfraction of the alkali that is subsequently used in the cooking stage isconsumed. The oxidising ability of the polysulphide, i.e. theyield-increasing effect, increases as the level of alkali increases.This means that it is also advantageous to add polysulphide with thewhite liquor at an early stage of the cooking process.

A further purpose is to obtain by the early addition of the alkali-richtreatment fluid at a moderate temperature and for a short retention timean efficient initial neutralisation of the chips and the dissolving ofreadily soluble lignin without the strength of the pulp beingsignificantly influenced. The longer impregnation processes and thefinal cooking process can, by the use of the method, take place at alower level of alkali, which gives an overall high pulp strength andhigh yield.

The invention can be used on both steam-phase digesters and on hydraulicdigesters; with inverted top separators, with downward-feeding topseparators and with types that lack a top separator; and it can be usedduring the production of cellulose pulp using both the sulphite processand the sulphate process. In the same way, deciduous wood, coniferouswood, annuals (such as bagasse, etc.) and others can constitute thesource of cellulose.

The invention can be used with chargewise cooking, in which the chipsare fed into a vessel in which sequential treatment subsequently takesplace on the stationary chips in the vessel.

DESCRIPTION OF DRAWINGS

FIG. 1 shows how the addition of white liquor is carried out accordingto the prior art.

FIG. 2 shows how the addition of white liquor is carried out accordingto the invention.

FIG. 3 shows the principle of application of the invention in a systemwith continuous cooking.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an overview of how cellulose pulp has been conventionallymanufactured with various basic cooking processes. The startingmaterial, often chipped wood, is fed into a chip hopper in which heatingof the chips by steam takes place (CSt/Chip Steaming). This often occursin several stages with steam (St/Steam) of successively increasing heatvalue (temperature) being used. The starting material is heated to theinterval 80±20° C., during a period of at least 5-60 minutes. Normally,toxic waste gases (Gas) are formed, which must be handled and destroyedin a waste gas system. After being heated by steam, the cellulosematerial is mixed or forms a sludge as a fluid/wood mixture through theaddition of fluid. The fluid that is added is obtained from a subsequenttreatment in the cooking stage, and is known as black liquor (BL/BlackLiquor), and often with the addition of white liquor/alkali (WL/WhiteLiquor). This fluid/wood mixture is introduced into a transfercirculation system, which often includes a pump or several pumps andsluice feeds (for example, a high-pressure tap), onwards to apressurised treatment vessel. Here, one treatment stage, BL-imp/BlackLiquor Impregnation, is shown, in which the cellulose material isimpregnated with black liquor having a predetermined level of residualalkali. In association with the input feed to this treatment stage, themajor fraction of the fluid is separated in the transfer circulationsystem and returned to the input, Csl/Feed, while new fluid in the formof black liquor and washing fluid, Wash Liq, is added.

Once the cellulose material has been treated with black liquor and hasconsumed the residual alkali in it, the remaining expended black liquoris withdrawn for recovery, BL-REC. The residual alkali level normallylies significantly under 15 g/l, typically under 8 g/l.

After the impregnation, which can occur in more than one stage withdifferent black liquors, the cooking itself is started. Two cookingstages are shown here, Cook1 and Cook2, where the first cooking stageCook1 can be what is known as a concurrent cooking stage, conventionallyat temperatures in the interval 150±20° C., where the alkali is added atthe start of the cooking stage and is allowed to accompany the chips ina continuous digester.

In older cooking systems, expended cooking fluid, black liquor, waswithdrawn from the cooking stage for recovery, Conv. & Mcc/ITC REC, andin this case no black liquor was returned to a previous black liquorimpregnation stage. The cooking in the second cooking stage Cook2 cantake place using a design with countercurrent flow, conventionally at atemperature in the interval 150±20° C. A certain amount of the alkalican at this stage be added at the bottom of the countercurrent flowzone, after which the cooking fluid passes in a flow that coursescounter to the flow of the chips. The chips normally have a retentiontime in the cooking zones Cook1/Cook2 that lies in the interval 40-200minutes.

Washing, also known as displacement, commences after the cooking stage,where dissolved lignin is washed from the cooked pulp in order to obtaina pulp with a value of kappa under 40, preferably a value of kappa under24. With respect to the addition of white liquor, WL, this was carriedout principally at the beginning of the cooking stage, and only verysmall charges, well under 50% and usually under 20%, of white liquorwere added at the impregnation stage before the cooking stage. Whenpolysulphide-rich white liquor was used to increase the yield, this wasadded during impregnation, at lower temperature, and was allowed toaccompany the chips to the cooking stage. The cooking stage was modifiedwhen black liquor impregnation was introduced such that a relativelyhigh level of residual alkali, normally around 20 g/l or higher, wasobtained in the black liquor withdrawn from the digester, which blackliquor was conveyed to the black liquor impregnation where the residualalkali was consumed down to a level that lay under 10 g/l, while themain fraction, greater than 50%, of the total charge of white liquor,WL, that was necessary for the process was still carried out at thecooking stage.

A preferred embodiment according to the invention is shown in FIG. 2,where the main part, more than 50%, of the total charge of the whiteliquor instead occurs at a pre-treatment stage that is located beforethe stage at which treatment with black liquor occurs.

The starting material (chips) is fed as previously shown to at least onestage at which heating of the chips by means of steam (CSt) occurs. Theheating of the starting material preferably takes place at a temperaturein the interval 80±20° C., during a period of at least 5-60 minutes.

After the heating with steam, the cellulose material is mixed or forms aslurry as a fluid/wood mixture by the addition of fluid (which has beenobtained from a subsequent treatment stage in the cooking stage) and amajor part of the total charge of fresh white liquor required for thecooking stage. At least 50% of the total charge of alkali that isrequired to cook the cellulose down to a kappa value lower than 40,preferably a value lower than 24, is added at this stage. As much as upto 100% of the total charge of fresh white liquor for the impregnationand the cooking stage can be added at this point.

This fluid/wood mixture with a high level of alkali is carried in atransfer circulation system in a conventional manner to a pressurisedtreatment vessel. One treatment stage, BL-imp/Black Liquor Impregnation,is shown here, in which the cellulose material is impregnated with blackliquor having a predetermined level of residual alkali.

In association with the input feed to this treatment stage, BL-imp, themajor fraction, at least 50% but up to 90%, of the fluid is separated inthe transfer circulation system, after which this fluid is added to thecellulose pulp in association with the cooking stage at the cookingtemperature. The possibility arises in this way for the white liquor toprecipitate its content of polysulphide onto the cellulose fibres at aprocess position, in this case during the transfer, where thetemperature is lower, which temperature is not sufficiently high for thepolysulphide to risk degradation to any major extent before theyield-increasing effect arises.

The alkali-rich fluid is replaced before the treatment with black liquorby black liquor.

Once the cellulose material has been treated with black liquor and hasconsumed the residual alkali in it, the remaining expended black liquoris withdrawn for recovery, BL-REC. Again in this case, the residualalkali level should normally lie significantly under 15 g/l, typicallyunder 8 g/l. After the impregnation, which can occur in more than onestage with different black liquors, the cooking itself is started. Twocooking stages are shown here, Cook1 and Cook2, where the first cookingstage Cook1 can be what is known as a concurrent cooking stage,conventionally at temperatures in the interval 150±20° C. When thealkali-rich fluid from the transfer circulation system has been added tothe cooking stage, the charge to the cooking stage of fresh alkali isreduced to the equivalent degree.

The alkali-rich fluid from the transfer circulation system and the smallamount of fresh alkali that is added at the start of he cooking stageare allowed to accompany the chips in a continuous digester.

The cooking in the second cooking stage Cook2 can take place using adesign with countercurrent flow, conventionally at a temperature in theinterval 150±° C. A certain amount of the alkali or the alkali-richwithdrawal from the impregnation stage can at this stage be added at thebottom of the countercurrent zone, after which the cooking fluid passesin a flow opposite to that of the chips. The chips normally have aretention time in the cooking zones Cook1/Cook2 that lies in theinterval 40-240 minutes, and preferably approximately 120±20° minutesper cooking zone.

Washing, also known as displacement, commences in a conventional wayafter the cooking stage, where dissolved lignin is washed from thecooked pulp in order to obtain a pulp with a value of kappa under 40,principally for coniferous wood, and preferably a value of kappa under24, principally for deciduous wood.

A system for the continuous cooking of cellulose pulp where the methodaccording to the invention can be applied is shown in FIG. 3. Chips arefed into a chip hopper 10 where the chips are heated by steam, St, withthe expulsion of waste gases, Gas. The chips heated in this way are thenfed to a chip chute 11 where a slurry with an appropriate fluid/woodratio is formed from the chips by the addition of white liquor, WL,possibly together with the addition off black liquor (not shown in thedrawing). The chips pass onwards from the bottom of the chip chute 11 bya high-pressure tap 12 through a transfer circulation system 13 a, 13 bto a pressurised treatment vessel 15 for black liquor impregnation. Thefluid that is added to the chip chute 11 and that accompanies the chipsin the line 13 a is separated to a large extent from the chips by a topseparator 14 and is returned to the high-pressure tap 12 through thereturn line 13 b. The alkali-rich transfer fluid is withdrawn in a flow30 to an extent that is equivalent to the addition of fresh white liquorat the chip chute 11 for later addition before the cooking stage. Theaddition of white liquor at this position in the system ensures arelatively short retention time is obtained at a moderate temperature,in the interval 60±40° C. for approximately 2-60 minutes, preferably2-10 minutes, which is the reason that the high level of alkali does nothave sufficient time to influence the strength of the pulp.

An impregnation with black liquor that has been added through the line31 and that has been withdrawn from the cooking stage through thewithdrawal filter 20 takes place in the treatment vessel 15. Theresidual alkali level in the black liquor in the line 31 normally liesconsiderably over 15 g/l.

Consumption of the residual alkali takes place in the treatment vessel15 and expended black liquor with a residual alkali level less than 10g/l is withdrawn from the filter 16 for onwards transport to therecovery system 32.

The chips are fed to the digester 19 after treatment with black liquorin the vessel 15 and it is appropriate that the alkali-rich fluid 30 isadded to the chips before the cooking stage in association with theoutput 17 from the treatment vessel 15. The starting material that hasbeen pre-treated in this way is continuously fed to the top of thedigester 19. After cooking in, for example, a first concurrent cookingstage and a final countercurrent cooking stage, the cooked pulp is fedout from the bottom of the digester and onwards to washing equipment, inthis case a pressurised-air diffuser, where the lignin that has beendissolved in the cooking phase is washed out in order to obtaincellulose pulp with a kappa value under 40, preferably with a kappavalue under 24.

Only those functions relevant to the invention are shown in FIG. 3.There may, for example, be several warming circuits or severalwithdrawal positions both in the impregnation vessel 15 and in thedigester 19. In the same way, several washing fluids or solvents can bewithdrawn at A, B or C in order to be added to the inlet, theimpregnation or the cooking phase, in order to establish the correctfluid/wood ratios in these zones.

If a chargewise cooking system is used for the manufacture of cellulosepulp, the alkali-rich treatment according to the invention can be placedbefore or after the impregnation with warm black liquor, where thechargewise cooking of the chips with which the vessel has been filledtakes place according to the sequence:

-   -   1) Filling of the vessel with chips.    -   2) Heating of the chips with steam.    -   3) Heating/impregnation with warm black liquor.    -   4) Heating/impregnation with hot black liquor.    -   5) Cooking with cooking liquor.    -   6) Washing with compression after the cooking stage, with the        expelled cooking fluid being stored in a tank for hot black        liquor.    -   7) Washing with compression after the previous stage with        washing fluid, where the fluid expelled first is stored in a        tank for warm black liquor.    -   8) Emptying of the cooked and washed chips.

The alkali-rich treatment can thus be placed before step 3 or after step3 in the sequence specified above, as an additional step or a step thatreplaces step 3 in which the chips are treated with the fresh whiteliquor.

The invention can be modified in several ways within the framework ofthe claims.

For example, the invention can also be used in a continuous digester inan upper treatment zone at an impregnation stage, which zone is limitedby a withdrawal filter in an upper part for treatment with alkali-richtreatment fluid, and at least one treatment zone for black liquor thatis located beneath it.

The alkali-rich fluid that has been withdrawn from the slurriedcellulose suspension before the black liquor impregnation stage can alsobe added at several positions in the cooking phase. For example, atleast a part of the alkali-rich treatment fluid can be added at thebottom of the digester at the end of a countercurrent zone. It can alsobe added in the middle of the cooking phase if all cooking zones in thedigester are concurrent zones.

The invention is most advantageous in pulp mills in whichpolysulphide-rich white liquor is produced for use in the cookingprocess, which polysulphide gives a significant increase in yield fromthe cooking stage if it can be used in an optimal manner without beingdegraded before its beneficial effects on the fibres are obtained. Thepolysulphide reacts much more rapidly than the alkali, which is why fulleffect of the polysulphide is obtained without any major consumption ofalkali.

1. A method for the manufacture of cellulose pulp, in which a startingmaterial, wood chips, is treated in several stages, in differenttreatment stages at successively increasing temperatures, where thefirst stage comprises warming the starting material with steam to afirst temperature, and in subsequent steps treatment of the startingmaterial with different impregnation fluids at successively increasingtemperatures in order to be finally cooked in an alkali cooking fluid ata predetermined cooking temperature in the interval 150±20° C., thechips are treated in a pre-treatment stage that lies before at least onetreatment with black liquor with a given level of residual alkali at atemperature that lies at least 20° C. under the cooking temperaturewhere an impregnation fluid in the pre-treatment stage is constituted byat least 50% of a total alkali charge that is required to cook thecellulose pulp, whereafter a main part of the impregnation fluid used inthe pre-treatment stage is withdrawn and is replaced before adding blackliquor for a black liquor treatment stage, and that the impregnationfluid withdrawn from the pre-treatment stage is added to the cellulosepulp in association with the cooking stage at the cooking temperature.2. The method according to claim 1, wherein the method further comprisesproviding the impregnation fluid in the pre-treatment stage by at least50% of the total charge of alkali required to impregnate and cooking thecellulose pulp down to a kappa value that lies under
 40. 3. The methodaccording to claim 1 wherein the method further comprises providing theimpregnation fluid in the pre-treatment stage by up to 100% of the totalcharge of alkali required to impregnate and cooking the cellulose pulpdown to a kappa value that lies under
 40. 4. The method according toclaim 1, wherein the method further comprises setting a temperature ofthe pre-treatment stage in the interval 60±40° C.
 5. The methodaccording to claim 4, wherein the method further comprises setting aretention time in the pre-treatment stage within an interval of 2-60minutes.
 6. The method according to claim 5, wherein the method furthercomprises providing the impregnation fluid withdrawn from thepre-treatment stage with a residual alkali level greater than 35 g/l. 7.The method according to claim 6, wherein the method further comprisestransferring the impregnation fluid withdrawn from the extra stage to aposition in a subsequent cooking stage with full cooking temperature. 8.The method according to claim 5, wherein the method further comprisesheating the starting material by means of steam before the pre-treatmentstage to a temperature in an interval of 60-100° C.
 9. The methodaccording to claim 1 wherein the method further comprises applying themethod in a continuous process with a continuous digester wherepre-treated starting material is fed continuously to a top of thedigester and where cooked cellulose is continuously fed out from abottom of the digester.
 10. The method according to claim 9, wherein themethod further comprises associating the pre-treatment stage with thetransfer of the starting material from a chip hopper through a transfercirculation system to a pressurised treatment vessel in which treatmentof the cellulose material takes place with black liquor with a givenlevel of residual alkali.
 11. The method according to claim 10, whereinthe method further comprises obtaining the black liquor with a givenlevel of residual alkali by withdrawal from the cooking process, wherethe black liquor has a level of residual alkali in an interval of 10-100g/l.
 12. The method according to claim 1 wherein the method furthercomprises applying the method in a chargewise process in which startingmaterial is fed to a vessel and in which different treatment fluids arethereafter used in a sequence in order to permeate the starting materialin the vessel until the starting material has been cooked to apredetermined kappa value.